They don't mix it. Blood isn't interchangeable, as they explain on that link they sometimes need to destroy individual donations after separation if tests find

They don't mix it. Blood isn't interchangeable, as they explain on that link they sometimes need to destroy individual donations after separation if tests find a problem (e.g. you unknowingly have HIV) so if they were mixing the blood they'd need to destroy far more when this happens. Perhaps more importantly match checks can't work effectively except on a single donation. If you're a non-emergency transfusion patient you'll be cross matched with the intended blood, which means they know before transfusing that it's definitely compatible. In fiction there are a handful of blood "types" and all the AB+ is identical, in reality it's more complicated than that, so if you've got enough time you always check and that means the donations must be from a single specific (even if anonymous) donor.